Personal communication networks are being deployed extensively world-wide using cellular mobile radio systems. There are now several cellular communication networks in operation. GSM900 (Global System for Mobile Communications) is the world's most widely used digital network and is in operation in over 100 countries around the world, predominantly in Europe and Asia Pacific. GSM1800 (DCS1800; PCN1800) operates at a higher frequency with respect to GSM900 and is in operation in Europe and Asia Pacific. GSM1900 (PCS1900) is used in the US and Canada and is scheduled for parts of Latin America, Australia and Africa. PDC (Personal Digital Cellular) is a Japanese digital network, AMPS (Advanced Mobile Phone System) is an analogue mobile phone network which is used mainly in the US and also Latin America, and Australia.
Earlier networks, still in operation, use analogue modulation formats for the radio air interface protocol. These analogue networks exhibit the problem of call saturation in high usage areas. To overcome this problem higher capacity air interface protocols using digital modulation format networks have been introduced in tandem, that is an area is covered by both systems. Nevertheless, since analogue networks have been established for a longer period, analogue networks may offer better coverage than digital networks. For example, in the United States and Canada the early standardised analogue network (AMPS) has reached a fairly universal coverage of the populated North American continent. The newer digital networks, however, tend to be deployed in areas of high usage. A result of this is that there are areas of digital network coverage overlaying a universal analogue network coverage.
Additionally, different air interface protocol standards of digital networks have been deployed regionally, since different telecommunications operators have developed their own protocols or have developed such protocols in line with national and sometimes international standards authorities, for example, the GSM protocol. Whilst it is reasonable to suppose that handsets operable for different radio communications protocols are similar from the users point of view, it is not possible, in particular, to use a digital mobile radio in an analogue cellular region and vice versa. This stems from the fact that whilst both types of handsets possess antennas, radio front end transmitter, receiver and baseband circuits, they operate on different air interface protocols which operate, inter alia at different radio carrier frequencies.
Therefore it can be seen that each individual personal communications system user will need to subscribe to two or more network providers for complete coverage. Consequently a mobile phone subscriber may require a handset that will not only function throughout the coverage area of a specific digital network, but also will have the capability to operate over an alternative network such as an analogue network.
The problem of implementing a dual mode handset has been considered to be surmountable by several different approaches; one solution uses two separate radio transceivers piggybacked and combined at the man-machine interface (keyboard and audio); a second solution uses two separate radio sections piggybacked and combined at the digital signal processing part of the radio transceiver,--applicants have a pending application relating to such a scheme, GB9603316.2. These two above approaches have problems in that the radio frequency signals are transmitted and received via an antenna. If the frequencies of operation are different, as indeed they will need to be, then two types of antenna will be necessary.
A number of dual band helical structures have been investigated at the Helsinki University of Technology, and these were presented at the 1996 IEEE VTC Conference. The helical structures presented are shown in FIG. 1. They consist of: (a) two helical antennas, one within the other; (b) a helical-monopole combination; and (c) a helical antenna combined with a wound monopole. The paper states that the dual frequency operation can be obtained from all three of the structures that are shown. Results for structure (a) state that it was tuned to the frequencies 1740 MHz and 900 MHz, and that 10 dB return loss bandwidths were obtained of 5.2% and 2.2% respectively. The dimensions for the antennas were D.sub.1 =6 mm, D.sub.2 =3 mm, D=5 mm, I.sub.h1 =12 mm, I.sub.h2 =14 mm, I.sub.m =39 mm, I.sub.h =13 mm, N.sub.1 =5, N.sub.2 =5, N.sub.3 =7, and I.sub.s =10 mm. Results for structure (b) state that it was tuned to the frequencies 1750 MHz and 894 MHz, and that 10 dB return loss bandwidths were obtained of 12% and 4.5% respectively. Structure (c) is simply a more compact version of (b), and not surprisingly has a narrower bandwidth. For the upper and lower bands, measured bandwidths of 11% and 2.9% were obtained where the overall structure height was 34 mm. Thus, in summary these antennas provide a bandwidth which is not sufficient for many radio applications, and also does not leave any margin for manufacturing tolerances.
A dual band external antenna is described by Ali et al in `A wide band dual meander sleeve antenna`, IEEE Antennas and Propagation Society International Symposium, 1995, vol.2 p.1124-7, 18-23 June 1995, Newport Beach, Calif., USA, and this is called the wide band dual meander sleeve antenna. This antenna is described as potentially useful as a low profile antenna for a dual mode handset. However, the results presented in the paper are for the case where the experimental antenna is mounted on a large ground plane (90 cm.sup.2) and as such would not be suitable for applications such as mobile telecommunications handsets. A single mode antenna small enough to be retracted within the casing of a handset has been proposed in various forms: the same cannot be said to be true for dual/multi-band antennas.
Applicants propose several types of dual/multi resonant antennas which provide sufficient bandwidth in the appropriate bands, as described in co-pending U.S. application Ser. Nos. 08/936314 and 08/943384. It is believed that these antennas may not be as compact as demanded by the trend for an overall decrease in mobile handset size.